Triple output photodetector current replicator

ABSTRACT

Methods and systems for replicating current outputs from a photodetector include using a transimpedance (TIA) amplifier to directly generate a TIA output voltage that is linear with optical power at the photodetector. Additionally, a first logarithmic amplifier is used to generate a first output voltage from the photodetector current and a second logarithmic amplifier is used to generate a second voltage from a copy of the photodetector current. The first voltage and the second voltage may be linear in some cases with the optical power in decibels.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to optical communicationnetworks and, more particularly, to a triple output photodetectorcurrent replicator.

Description of the Related Art

Telecommunication, cable television and data communication systems useoptical networks to rapidly convey large amounts of information betweenremote points. In an optical network, information is conveyed in theform of optical signals through optical fibers. Optical networks mayalso include various network elements, such as amplifiers, dispersioncompensators, multiplexer/demultiplexer filters, wavelength selectiveswitches, optical switches, couplers, etc. configured to perform variousoperations within the network.

In particular, optical networks may be reconfigured to transmitdifferent individual channels using, for example, optical add-dropmultiplexers (OADMs). In this manner, individual channels (e.g.,wavelengths) may be added or dropped at various points along an opticalnetwork, enabling a variety of network configurations and topologies.However, such network reconfiguration events may result in powertransients among the surviving channels. In particular, steady-stategain offset as a result of network reconfiguration may result inundesired variations in signal power and optical signal to noise ratio(OSNR) in an optical network.

SUMMARY

In one aspect, a disclosed method is for current replication. The methodmay include receiving, at a photodetector, an optical signal having aplurality of wavelengths and an optical power. Responsive to the opticalsignal received, the method may include generating a photodetectorcurrent at the photodetector. In the method, the photodetector may bereverse biased with a bias voltage. The method may further includeoutputting the photodetector current from an anode of the photodetectorto a first logarithmic amplifier to generate a first output voltage thatis linear with the optical power in decibels (dB). The method may alsoinclude generating a copy current using a transimpedance amplifier (TIA)having a first feedback resistor and a second feedback resistor. In themethod, a negative terminal of the transimpedance amplifier may be acathode of the photodetector, and a TIA output voltage that is linearwith the optical power may be generated. The method may still furtherinclude outputting the copy current from a positive terminal of thetransimpedance amplifier to a second logarithmic amplifier to generate asecond voltage that is linear with the optical power in dB.

In any of the disclosed embodiments of the method, the photodetector maybe a photodiode. In any of the disclosed embodiments of the method, aratio of the first feedback resistor to the second feedback resistor mayscale the copy current with respect to the photodetector current. In themethod, when the first feedback resistor equals the second feedbackresistor, the copy current may equal the photodetector current.

In any of the disclosed embodiments, the method may include applying acorrection current at the anode of the photodetector to offset the firstvoltage. In the method, the correction current may not affect the TIAoutput voltage and may not affect the second voltage.

In any of the disclosed embodiments of the method, the bias voltage maybe a differential voltage, while the method may include applying anegative pole of the bias voltage to a reference voltage input at thefirst logarithmic amplifier, and applying a positive pole of the biasvoltage to a reference voltage input at the second logarithmicamplifier.

In another aspect, a disclosed circuit is a photodetector currentreplicator circuit. The circuit may include a photodetector forreceiving an optical signal having a plurality of wavelengths and anoptical power. In the circuit, the photodetector generates aphotodetector current when reverse biased with a bias voltage. Thecircuit may further include a first logarithmic amplifier for receivingthe photodetector current from an anode of the photodetector andgenerating a first output voltage that is linear with the optical powerin decibels (dB). The circuit may also include a transimpedanceamplifier (TIA) to generate a copy current and having a first feedbackresistor and a second feedback resistor. In the circuit, a negativeterminal of the transimpedance amplifier may be a cathode of thephotodetector, and a TIA output voltage that is linear with the opticalpower may be generated. The circuit may still further include a secondlogarithmic amplifier for receiving the copy current from a positiveterminal of the transimpedance amplifier and generating a second voltagethat is linear with the optical power in dB.

In any of the disclosed embodiments of the circuit, the photodetectormay be a photodiode. In any of the disclosed embodiments of the circuit,a ratio of the first feedback resistor to the second feedback resistormay scale the copy current with respect to the photodetector current. Inthe circuit, when the first feedback resistor equals the second feedbackresistor, the copy current may equal the photodetector current.

In any of the disclosed embodiments, the circuit may include acorrection current applied at the anode of the photodetector to offsetthe first voltage. In the circuit, the correction current may not affectthe TIA voltage and may not affect the second voltage.

In any of the disclosed embodiments of the circuit, the bias voltage maybe a differential voltage, while a negative pole of the bias voltage maybe applied to a reference voltage input at the first logarithmicamplifier, and a positive pole of the bias voltage may be applied to areference voltage input at the second logarithmic amplifier.

Additional disclosed aspects of a triple output photodetector currentreplicator include an optical communication system and an opticalamplifier used in the optical communication system, as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of selected elements of an embodiment of anoptical network;

FIG. 2 is a block diagram of selected elements of an embodiment of atriple output photodetector current replicator; and

FIG. 3 is a flow diagram of selected elements of an embodiment of amethod for triple output photodetector current replication.

DESCRIPTION OF PARTICULAR EMBODIMENT(S)

In the following description, details are set forth by way of example tofacilitate discussion of the disclosed subject matter. It should beapparent to a person of ordinary skill in the field, however, that thedisclosed embodiments are exemplary and not exhaustive of all possibleembodiments.

Throughout this disclosure, a hyphenated form of a reference numeralrefers to a specific instance of an element and the un-hyphenated formof the reference numeral refers to the element generically orcollectively. Thus, as an example (not shown in the drawings), widget“12-1” refers to an instance of a widget class, which may be referred tocollectively as widgets “12” and any one of which may be referred togenerically as a widget “12”. In the figures and the description, likenumerals are intended to represent like elements.

Turning now to the drawings, FIG. 1 illustrates an example embodiment ofoptical network 101, which may represent an optical communicationsystem. Optical network 101 may include one or more optical fibers 106configured to transport one or more optical signals communicated bycomponents of optical network 101. The network elements of opticalnetwork 101, coupled together by fibers 106, may comprise one or moretransmitters 102, one or more multiplexers (MUX) 104, one or moreoptical amplifiers 108, one or more optical add/drop multiplexers (OADM)110, one or more demultiplexers (DEMUX) 105, and one or more receivers112.

Optical network 101 may comprise a point-to-point optical network withterminal nodes, a ring optical network, a mesh optical network, or anyother suitable optical network or combination of optical networks.Optical fibers 106 comprise thin strands of glass capable ofcommunicating the signals over long distances with very low loss.Optical fibers 106 may comprise a suitable type of fiber selected from avariety of different fibers for optical transmission.

Optical network 101 may include devices configured to transmit opticalsignals over optical fibers 106. Information may be transmitted andreceived through optical network 101 by modulation of one or morewavelengths of light to encode the information on the wavelength. Inoptical networking, a wavelength of light may also be referred to as achannel. Each channel may be configured to carry a certain amount ofinformation through optical network 101.

To increase the information capacity and transport capabilities ofoptical network 101, multiple signals transmitted at multiple channelsmay be combined into a single wideband optical signal. The process ofcommunicating information at multiple channels is referred to in opticsas wavelength division multiplexing (WDM). Coarse wavelength divisionmultiplexing (CWDM) refers to the multiplexing of wavelengths that arewidely spaced having low number of channels, usually greater than 20 nmand less than sixteen wavelengths, and dense wavelength divisionmultiplexing (DWDM) refers to the multiplexing of wavelengths that areclosely spaced having large number of channels, usually less than 0.8 nmspacing and greater than forty wavelengths, into a fiber. WDM or othermulti-wavelength multiplexing transmission techniques are employed inoptical networks to increase the aggregate bandwidth per optical fiber.Without WDM, the bandwidth in optical networks may be limited to thebit-rate of solely one wavelength. With more bandwidth, optical networksare capable of transmitting greater amounts of information. Opticalnetwork 101 may be configured to transmit disparate channels using WDMor some other suitable multi-channel multiplexing technique, and toamplify the multi-channel signal.

Optical network 101 may include one or more optical transmitters (Tx)102 configured to transmit optical signals through optical network 101in specific wavelengths or channels. Transmitters 102 may comprise asystem, apparatus or device configured to convert an electrical signalinto an optical signal and transmit the optical signal. For example,transmitters 102 may each comprise a laser and a modulator to receiveelectrical signals and modulate the information contained in theelectrical signals onto a beam of light produced by the laser at aparticular wavelength, and transmit the beam for carrying the signalthroughout optical network 101.

Multiplexer 104 may be coupled to transmitters 102 and may be a system,apparatus or device configured to combine the signals transmitted bytransmitters 102, e.g., at respective individual wavelengths, into a WDMsignal.

Optical amplifiers 108 may amplify the multi-channeled signals withinoptical network 101. Optical amplifiers 108 may be positioned before andafter certain lengths of fiber 106. Optical amplifiers 108 may comprisea system, apparatus, or device configured to amplify optical signals.For example, optical amplifiers 108 may comprise an optical repeaterthat amplifies the optical signal. This amplification may be performedwith opto-electrical or electro-optical conversion. In some embodiments,optical amplifiers 108 may comprise an optical fiber doped with arare-earth element to form a doped fiber amplification element. When asignal passes through the fiber, external energy may be applied in theform of a pump signal to excite the atoms of the doped portion of theoptical fiber, which increases the intensity of the optical signal. Asan example, optical amplifiers 108 may comprise an erbium-doped fiberamplifier (EDFA).

OADMs 110 may be coupled to optical network 101 via fibers 106. OADMs110 comprise an add/drop module, which may include a system, apparatusor device configured to add or drop optical signals (i.e., at individualwavelengths) from fibers 106. After passing through an OADM 110, anoptical signal may travel along fibers 106 directly to a destination, orthe signal may be passed through one or more additional OADMs 110 andoptical amplifiers 108 before reaching a destination.

In certain embodiments of optical network 101, OADM 110 may represent areconfigurable OADM (ROADM) that is capable of adding or droppingindividual or multiple wavelengths of a WDM signal. The individual ormultiple wavelengths may be added or dropped in the optical domain, forexample, using a wavelength selective switch (WSS) (not shown) that maybe included in a ROADM.

As shown in FIG. 1, optical network 101 may also include one or moredemultiplexers 105 at one or more destinations of network 101.Demultiplexer 105 may comprise a system apparatus or device that acts asa demultiplexer by splitting a single composite WDM signal intoindividual channels at respective wavelengths. For example, opticalnetwork 101 may transmit and carry a forty (40) channel DWDM signal.Demultiplexer 105 may divide the single, forty channel DWDM signal intoforty separate signals according to the forty different channels.

In FIG. 1, optical network 101 may also include receivers 112 coupled todemultiplexer 105. Each receiver 112 may be configured to receiveoptical signals transmitted at a particular wavelength or channel, andmay process the optical signals to obtain (e.g., demodulate) theinformation (i.e., data) that the optical signals contain. Accordingly,network 101 may include at least one receiver 112 for every channel ofthe network.

Optical networks, such as optical network 101 in FIG. 1, may employmodulation techniques to convey information in the optical signals overthe optical fibers. Such modulation schemes may include phase-shiftkeying (PSK), frequency-shift keying (FSK), amplitude-shift keying(ASK), and quadrature amplitude modulation (QAM), among other examplesof modulation techniques. In PSK, the information carried by the opticalsignal may be conveyed by modulating the phase of a reference signal,also known as a carrier wave, or simply, a carrier. The information maybe conveyed by modulating the phase of the signal itself using two-levelor binary phase-shift keying (BPSK), four-level or quadraturephase-shift keying (QPSK), multi-level phase-shift keying (M-PSK) anddifferential phase-shift keying (DPSK). In QAM, the information carriedby the optical signal may be conveyed by modulating both the amplitudeand phase of the carrier wave. PSK may be considered a subset of QAM,wherein the amplitude of the carrier waves is maintained as a constant.Additionally, polarization division multiplexing (PDM) technology mayenable achieving a greater bit rate for information transmission. PDMtransmission comprises modulating information onto various polarizationcomponents of an optical signal associated with a channel. Thepolarization of an optical signal may refer to the direction of theoscillations of the optical signal. The term “polarization” maygenerally refer to the path traced out by the tip of the electric fieldvector at a point in space, which is perpendicular to the propagationdirection of the optical signal.

In an optical network, such as optical network 101 in FIG. 1, it istypical to refer to a management plane, a control plane, and a transportplane (sometimes called the physical layer). A central management host(not shown) may reside in the management plane and may configure andsupervise the components of the control plane. The management planeincludes ultimate control over all transport plane and control planeentities (e.g., network elements). As an example, the management planemay consist of a central processing center (e.g., the central managementhost), including one or more processing resources, data storagecomponents, etc. The management plane may be in electrical communicationwith the elements of the control plane and may also be in electricalcommunication with one or more network elements of the transport plane.The management plane may perform management functions for an overallsystem and provide coordination between network elements, the controlplane, and the transport plane. As examples, the management plane mayinclude an element management system (EMS), which handles one or morenetwork elements from the perspective of the elements, a networkmanagement system (NMS), which handles many devices from the perspectiveof the network, and an operational support system (OSS), which handlesnetwork-wide operations.

Modifications, additions or omissions may be made to optical network 101without departing from the scope of the disclosure. For example, opticalnetwork 101 may include more or fewer elements than those depicted inFIG. 1. Also, as mentioned above, although depicted as a point-to-pointnetwork, optical network 101 may comprise any suitable network topologyfor transmitting optical signals such as a ring, a mesh, or ahierarchical network topology.

As discussed above, the amount of information that may be transmittedover an optical network may vary with the number of optical channelscoded with information and multiplexed into one signal. Accordingly, anoptical fiber employing a WDM signal may carry more information than anoptical fiber that carries information over a single channel. Besidesthe number of channels and number of polarization components carried,another factor that affects how much information can be transmitted overan optical network may be the bit rate of transmission. The higher thebit rate, the greater the transmitted information capacity. Achievinghigher bit rates may be limited by the availability of wide bandwidthelectrical driver technology, digital signal processor technology andincrease in the required OSNR for transmission over optical network 101.

In operation of optical network 101, reconfiguration of the opticalsignals to add or drop individual channels may be performed at OADMs110. Under such add/drop cases, the surviving channels maysystematically be subjected to power transients that result in undergain or over gain. The under or over gain of the surviving channels mayaccumulate rapidly along cascaded optical amplifiers 108 as thistransient gain offset may lead to undesirable variation in output signalpower and received OSNR. In particular, as higher bit rates, for exampleup to 100 gigabits per second, are used for transmission over opticalnetwork 101, the received OSNR to achieve such higher bit rates may bereduced due to transient gain (TG) effects. In addition to networkthroughput, variation in OSNR due to transient gain effects mayconstrain a transmission distance (i.e., reach) of at least certainportions of optical network 101.

In optical network 101, TG effects may be compensated or minimizeddynamically using fast loop control, for example, in optical amplifiers108. However, in typical optical amplifiers, fast loop control may belimited by the bandwidth response of certain components and signallevels. Specifically, the optical amplifier may include respectiveoptical taps at an optical input and at an optical output for measuringinput signal levels and output signal levels in order to regulate thegain of the optical amplifier. The optical taps enable a photodetector,such as a photodiode, to generate an electrical signal indicative of thesignal level (or optical power level).

In a typical optical amplifier, the outputs of the photodetectors may befed to different elements and in some cases, multiple current outputsare used for various purposes, such as gain control regulation.Additional voltage outputs that are indicative of the outputphotodetector current may also be used for various purposes in anoptical amplifier. In some embodiments, the photodetector current is feddirectly into a logarithmic amplifier (also referred to as a “log amp”)to generate a voltage that is linear in decibels (dB) with the outputphotodetector current. Because the optical amplifier generally is usedto increase the signal power, a current level at an input of the opticalamplifier may be smaller than a current level at an output of theoptical amplifier. To generate the multiple current outputs, a currentreplicator (sometimes referred to as a current mirror) is used. However,typical current replicators may come with certain disadvantages whenused in an optical amplifier. Specifically, typical current replicatorsmay not exhibit sufficient bandwidth at low input currents for gainregulation in an optical amplifier where currents as low as 100 nA mayoccur and where gain regulation bandwidth may be desired at 1 MHz orgreater.

As will be described in further detail herein, a triple outputphotodetector current replicator is disclosed that enables generation oftwo independent current sources. The triple output photodetector currentreplicator disclosed herein may enable a reverse biasing of aphotodetector while generating the two independent output currents. Thetriple output photodetector current replicator disclosed herein mayenable independent manipulation, such as biasing, offsetting,compensation, adding, subtracting, etc., of the two independent outputcurrents of the photodetector current. The triple output photodetectorcurrent replicator disclosed herein may provide a first voltage outputand a second voltage output that are respectively linear in dB with thetwo independent output currents. The triple output photodetector currentreplicator disclosed herein may also provide a third voltage output thatis linear with the photodetector current.

Turning now to FIG. 2, an example embodiment of triple outputphotodetector current replicator 200 is illustrated in block diagramformat. As shown, triple output photodetector current replicator 200 maybe used in an embodiment of optical amplifier 108 (see FIG. 1). Opticalamplifier 108 may be based on a doped fiber amplification element, suchas an erbium doped fiber, and may include a gain control circuit thatincludes triple output photodetector current replicator 200.

As shown in FIG. 2, triple output photodetector current replicator 200may represent an electronic device comprising various components andsignals between the components, which is shown as a schematic circuitdiagram. It is noted that arrows depicted in signal lines in FIG. 2 areintended to show information flow and may not necessarily represent adirection of transmission of a corresponding signal media (e.g.,transmission of an optical signal or an electrical current). As shown,photodetector 220 is a photodiode that is reverse biased. However, othertypes of photodetectors and bias arrangements may be used in variousembodiments.

In FIG. 2, triple output photodetector current replicator 200 may bebiased by applying V_(−bias) and V_(+bias) to respective referencevoltage inputs (V_(REF)) of log amp 210-1 and 210-2. In log amps 210,the voltage applied to V_(REF) will also appear as a voltage at thecurrent input I_(IN). As a result, V_(−bias) appears at node 214 (atcurrent input I_(IN) of log amp 210-1/anode of photodetector 220), whileV_(+bias) appears at positive terminal of a transimpedance amplifier 204at node 218 (at current input I_(IN) of log amp 210-2), which isreplicated at negative terminal of TIA 204 at node 212, and whichreverse biases photodetector 220.

Then, triple output photodetector current replicator 200 may receiveoptical signal 206 at photodetector 220. Optical signal 206 may includea plurality of wavelengths and may exhibit an optical power that variesover time. The wavelengths in optical signal 206 may representindividual channels, as described previously. In various embodiments,optical signal 206 may be a WDM signal. Optical signal 206 may bediverted from a transmission line, such as by using an optical splitteror an optical tap (not shown). As photons from optical signal 206 reachphotodetector 220 in the reverse biased state, a photodetector currentI_(PD) 216-1 is generated. In other words, photodetector 220 operates asa current source while I_(PD) 216-1 carries signal information that iscontained in optical signal 206. When optional correction currentI_(CORR) 222-1 (shown as a dashed arrow) is not present, I_(PD) 216-1flows via node 214 to current input I_(IN) of log amp 210-1, resultingin generation of a first voltage V₁ that is linear with optical signal206 in dB. Concurrently, I_(PD) 216-1 flows at node 212 (thephotodetector cathode/negative terminal to TIA amplifier 204) acrossresistor R1 resulting in generation of a TIA voltage V_(TIA) at anoutput terminal of transimpedance amplifier 204. Transimpedanceamplifier 204 may be an operational amplifier that is configured asshown in FIG. 2. Furthermore, I_(PD) 216-1 is replicated across resistorR2 and at node 218 (positive terminal to TIA amplifier 204) a copycurrent I_(COPY) 216-2 is generated. When optional correction currentI_(CORR) 222-2 (shown as a dashed arrow) is not present, I_(COPY) 216-2flows to current input I_(IN) of log amp 210-2, resulting in generationof a second voltage V₂ that is linear with the optical power of opticalsignal 206 in dB.

In triple output photodetector current replicator 200, a ratio of R1 toR2 may be used to scale I_(PD) 216-1 with respect to I_(COPY) 216-2since (I_(PD)*R1)=(I_(COPY)* R2)=V_(TIA). It will be understood thatwhen R1 equals R2, then I_(PD) 216-1 equals I_(COPY) 216-2.

Furthermore, as a result of the circuit configuration of triple outputphotodetector current replicator 200, I_(PD) 216-1 and I_(COPY) 216-2may be independently manipulated without affecting each other andwithout affecting TIA voltage V_(TIA). For example, I_(CORR) 222-1,which is an optional current, may be applied to node 214 withoutaffecting I_(PD) 216-1 or I_(COPY) 216-2, but rather, only affects thecurrent input I_(N) at log amp 210-1. In this manner, I_(CORR) 222-1 maybe used to generate a desired offset signal, for example, to compensatefor amplified spontaneous emission (ASE) or other types of backgroundnoise or white noise. In another example, I_(CORR) 222-2, which is alsoan optional current, may be applied at node 218 without affecting I_(PD)216-1 or I_(COPY) 216-2, but rather, only affects the current inputI_(IN) at log amp 210-2.

Referring now to FIG. 3, a block diagram of selected elements of anembodiment of method 300 for triple output photodetector currentreplication is depicted in flowchart form. Method 300 may be performedusing replicator circuit 200 (see FIG. 2), which may be employed inoptical amplifier 108 (see FIG. 1). It is noted that certain operationsdescribed in method 300 may be optional or may be rearranged indifferent embodiments.

Method 300 may begin at step 302 by receiving, at a photodetector, anoptical signal having a plurality of wavelengths and an optical power.At step 304, a photodetector current may be generated at thephotodetector when the photodetector is reverse biased with a biasvoltage. At step 306, the photodetector current may be output from ananode of the photodetector to a first logarithmic amplifier to generatea first output voltage that is linear with the optical power in dB. Atstep 308, a copy current may be generated using a transimpedanceamplifier having a first feedback resistor and a second feedbackresistor, such that a negative terminal of the transimpedance amplifieris a cathode of the photodetector, and a TIA output voltage (V_(TIA))that is linear with the optical power is generated. At step 310, thecopy current may be output from a positive terminal of thetransimpedance amplifier to a second logarithmic amplifier to generate asecond output voltage that is linear with the optical power in dB.

As disclosed herein, methods and systems for replicating current outputsfrom a photodetector include using a transimpedance (TIA) amplifier todirectly generate a TIA voltage that is linear with optical power at thephotodetector. Additionally, a first logarithmic amplifier is used togenerate a first output voltage from the photodetector current and asecond logarithmic amplifier is used to generate a second output voltagefrom a copy of the photodetector current. The first voltage and thesecond voltage may be linear in some cases with the optical power indecibels.

The above disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosure. Thus, to the maximumextent allowed by law, the scope of the present disclosure is to bedetermined by the broadest permissible interpretation of the followingclaims and their equivalents, and shall not be restricted or limited bythe foregoing detailed description.

What is claimed is:
 1. A method for current replication, the methodcomprising: receiving, at a photodetector, an optical signal having aplurality of wavelengths and an optical power; responsive to the opticalsignal received, generating a photodetector current at thephotodetector, wherein the photodetector is reverse biased with a biasvoltage; outputting the photodetector current from an anode of thephotodetector to a first logarithmic amplifier to generate a firstoutput voltage that is linear with the optical power in decibels (dB);generating a copy current using a transimpedance amplifier (TIA) havinga first feedback resistor and a second feedback resistor, wherein anegative terminal of the transimpedance amplifier is a cathode of thephotodetector, and wherein a TIA output voltage that is linear with theoptical power is generated; and outputting the copy current from apositive terminal of the transimpedance amplifier to a secondlogarithmic amplifier to generate a second voltage that is linear withthe optical power in dB.
 2. The method of claim 1, wherein thephotodetector is a photodiode.
 3. The method of claim 1, wherein a ratioof the first feedback resistor to the second feedback resistor scalesthe copy current with respect to the photodetector current.
 4. Themethod of claim 3, wherein, when the first feedback resistor equals thesecond feedback resistor, the copy current equals the photodetectorcurrent.
 5. The method of claim 1, further comprising: applying acorrection current at the anode of the photodetector to offset the firstvoltage, wherein the correction current does not affect the TIA voltageand does not affect the second voltage.
 6. The method of claim 1,wherein the bias voltage is a differential voltage, and furthercomprising: applying a negative pole of the bias voltage to a referencevoltage input at the first logarithmic amplifier; and applying apositive pole of the bias voltage to a reference voltage input at thesecond logarithmic amplifier.
 7. A photodetector current replicatorcircuit comprising: a photodetector for receiving an optical signalhaving a plurality of wavelengths and an optical power, wherein thephotodetector generates a photodetector current when reverse biased witha bias voltage; a first logarithmic amplifier for receiving thephotodetector current from an anode of the photodetector and generatinga first output voltage that is linear with the optical power in decibels(dB); a transimpedance amplifier (TIA) to generate a copy current andhaving a first feedback resistor and a second feedback resistor, whereina negative terminal of the transimpedance amplifier is a cathode of thephotodetector, and wherein a TIA output voltage that is linear with theoptical power is generated; and a second logarithmic amplifier forreceiving the copy current from a positive terminal of thetransimpedance amplifier and generating a second voltage that is linearwith the optical power in dB.
 8. The circuit of claim 7, wherein thephotodetector is a photodiode.
 9. The circuit of claim 7, wherein aratio of the first feedback resistor to the second feedback resistorscales the copy current with respect to the photodetector current. 10.The circuit of claim 9, wherein, when the first feedback resistor equalsthe second feedback resistor, the copy current equals the photodetectorcurrent.
 11. The circuit of claim 8, further comprising: a correctioncurrent applied at the anode of the photodetector to offset the firstvoltage, wherein the correction current does not affect the TIA outputvoltage and does not affect the second voltage.
 12. The circuit of claim8, wherein the bias voltage is a differential voltage, and wherein: anegative pole of the bias voltage is applied to a reference voltageinput at the first logarithmic amplifier; and a positive pole of thebias voltage is applied to a reference voltage input at the secondlogarithmic amplifier.